This invention relates to an combined cycle power generation system and method utilizing a coal fired gasifier and, more particularly, to such a system and method in which synthesis gas is produced and is used to provide heat for a steam cycle and to drive a gas turbine.
In order to reduce sulfur dioxide emission levels in fossil fuel power stations, a generation of stack gas clean up equipment has evolved which removes, or scrubs, sulfur dioxide from the steam generator flue gases prior to their release into the atmosphere. However, this is expensive largely due to the equipment required and the high volume of gases that must be processed.
As a result, a gasification process has evolved in which a carbonaceous material, such as particulate coal, is gasified in, for instance, an entrained flow gasifier. A combustible synthesis gas ("syngas") is produced by the gasification of the carbonaceous material. Desulfurization is accomplished after the syngas is cooled sufficiently using well known syngas scrubbers. The gas is then passed to a gas turbine combustor and then to the gas turbine expander which drives an electric generator.
In the production of electric power, improved efficiencies can be achieved by utilizing a combined gas turbine and steam turbine cycle. In these arrangements the syngas is used to drive a gas turbine as described above while a steam turbine cycle is carried out with steam which is produced from cooling the syngas and the combustion gas leaving the gas turbine.
However, in these arrangements the gasification of the carbonaceous material produces a bulk ash which, due to the fact that it is in an environment in which the temperature is above it's softening temperature, becomes sticky and molten causing fouling on reactor and heat exchanger surfaces. The soft ash particles are also extremely difficult to handle and transport which is a major problem since heat must subsequently be removed from the particles and from a mixture of syngas and the particles.